Problems with updating magic-string

[dummdidumm]

Wanted to give updating magic-string in svelte2tsx a shot but it seems that Rich-Harris/magic-string#261 in some way broke our code generation.

The minimum reproduction is this:
Bump magic-string in svelte2tsx to the latest version, then try renaming blubb in this file and you'll see it removes the braces:

<script>
    let blubb;
</script>
{blubb}

I'm not sure yet if this is because we're doing something unsupported/unintended or if this is a bug within magic-string.